This invention relates to a light source device, an illuminator for a projector of a ray splitting type using a dichroic mirror for color separation, and an image projector provided with such an illuminator.
As optical image projectors, there have been marketed liquid crystal projectors. A liquid crystal projector acts as a means for displaying a large image. The projector enlargedly projects an optical image on a screen via a projection lens. The optical image emerges from liquid crystal panels after modulation of the luminance of illumination light in accordance with an image signal.
A schematic construction of an illumination optical system of such a liquid crystal projector is described with reference to FIG. 6. In FIG. 6, white light is irradiated from lamps (light sources) 51, 52 disposed in a vertical direction. Parallel light rays are directed onto a first lens array 53. In this construction, parallel light rays of the lamp 51 are substantially incident onto an upper portion of the first lens array 53 while parallel light rays of the lamp 52 are substantially incident onto a lower portion of the first lens array 53.
These parallel light rays are further bundled by the first lens array 53 and a second lens array 54 each including a plurality of lenses, and are incident onto a display screen of a liquid crystal panel 57 in a superposing manner via an integrating lens 55 and a field lens 56. The first lens array 53, the second lens array 54, and the integrating lens 55 constitute an optical integrator.
Dichroic mirrors 58, 59 constituting a color separating optical system are disposed between the optical integrator and the field lens 56 to separate the white light rays into color light rays in the wavelength range of Red (R), Green (G), and Blue (B). Generally, spectral characteristics of the dichroic mirror 58 (59) exhibit an incident angle dependency as shown in FIGS. 7 and 8. In other words, the wavelength range of transmitted light or reflected light differs depending on the incident angle of light. Hereafter, this characteristic is referred to as "incident angle characteristic".
In the arrangement of the conventional liquid crystal projector, the two lamps 51, 52 are disposed immediately before the optical integrator. As shown in FIG. 6, parallel light rays of the upper lamp 51 are incident onto substantially the upper portion of the first lens array 53 while parallel light rays of the lower lamp 52 are incident onto substantially the lower portion of the first lens array 53. In this case, observing the light rays of the upper lamp 51 advancing to a center of the liquid crystal panel 57 or represented by the solid lines, these light rays are incident on the dichroic mirrors 58, 59 at an angle of (45.degree.+.alpha.). On the other hand, observing the light rays of the lower lamp 52 advancing to the center of the liquid crystal panel 57 or represented by the broken lines, these light rays are incident on the dichroic mirrors 58, 59 at an angle of (45.degree.-.alpha.).
As mentioned above, light rays of the upper lamp 51 are incident on the dichroic mirrors 58, 59 at an angle different from those irradiated of the lower lamp 52. Further, the dichroic mirrors 58, 59 have the incident angle characteristics. Accordingly, the problem is likely to occur that the display screen of the liquid crystal panel 57 is illuminated at a different color tone. For this reason, in the case where one of the lamps used, such as, the case where the lamps 51, 52 are alternately used, the case where one of the lamps 51, 51 should keep turning on after the other lamp is turned off under a certain circumstance during the use of both lamps 51, 52, or in the case where the luminance of the lamps 51, 52 differs from each other, the color tone of an optical image projected on the screen are likely to vary.
Also, it should be noted that unless the optical integrator is provided, it becomes more difficult to attain a luminance uniformity on the liquid crystal panel 57.